Supplemental Disclosures to the Consolidated Statements of Cash Flows (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2019	Jun. 30, 2019	Jun. 30, 2018
Supplemental Cash Flow Information [Abstract]
Deferred and contingent consideration from prior years acquisition		$ 39	$ 85
Deferred purchase consideration from prior years' acquisitions			53
Contingent consideration from prior year's acquisitions		16	32
Net charge for adjustments related to acquisition related accounts		20	11
Payment of contingent consideration		29	9
Non-cash issuance of common stock		162	128
Stock-based compensation expense, equity awards		$ 117	$ 99
Right-of-use asset, noncash	$ 1,700
Right-to-use liability, noncash	$ 1,900